Many different processes and compositions have been proposed for the reduction of nitrogen oxides in an effluent, each process and/or composition providing improved NO.sub.x reduction. For instance, in U.S. Pat. No. 3,900,554, Lyon discloses reducing nitrogen monoxide (NO) in a combustion effluent using ammonia and specified ammonia precursors or their aqueous solutions which are injected into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600.degree. F. to 2000.degree. F. Lyon also suggests the use of reducing agents, such as hydrogen or various hydrocarbons, to permit the effective use of ammonia at effluent temperatures as low as 1300.degree. F. Lyon points out that at temperatures above 2000.degree. F., the use of ammonia as disclosed in the patent is counterproductive, increasing NO rather than decreasing it.
Unfortunately, access to the effluent stream of a large industrial boiler at a location where the temperature is from 1600.degree. F. to 2000.degree. F. is often not possible without major redesign of the boiler due to exterior water jacketting and interior water tubes. The use of hydrogen to permit operation at temperatures as low as 1300.degree. F. is prohibitively expensive. Thus, the teaching of Lyon is not usable for most large utility boilers.
Dean et al., using the Lyon method as a starting point, disclose a method for reducing NO.sub.x concentrations in an effluent having a temperature of from 1880.degree. F. to 2920.degree. F., in U.S. Pat. No. 4,624,840. The Dean et al. process involves the injection of ammonia into the effluent stream at a point where the effluent is rapidly cooling. As discussed above, though, injection at the most desirable locations is frequently not possible in large utility boilers. Further, the Dean et al. process requires specific effluent cooling rate measurements, appropriate software and the solution of a complex set of simultaneous equations for determining the amount of ammonia to be injected and the location of the proper injection point to achieve NO reduction, a significant disadvantage over processes which can be operated without the need for such extreme procedures.
Bowers, in copending and commonly assigned U.S. patent application Ser. No. 811,532, filed Dec. 20, 1985, effects the reduction in NO.sub.x concentrations in the effluent from the combustion of a carbonaceous fuel without the use of ammonia with its attendant disadvantages through the use of a solution comprising additives selected from the group consisting of guanidine, guanidine carbonate, biguanidine, guanylurea sulfate, melamine, dicyandiamide, calcium cyanamide, biuret, 1,1'-azobisformamide, methylol urea, methylol urea-urea condensation product, dimethylol urea, methyl urea, dimethyl urea, hexamethylenetetramine and mixtures thereof. In copending and commonly assigned U.S. patent application Ser. No. 784,828, filed Oct. 4, 1985, now U.S. Pat. No. 4,719,092 Bowers discloses the use of an aqueous urea solution, which also comprises an oxygenated hydrocarbon, in the reduction of nitrogen oxides in an effluent.
Although many of the above-described compositions are effective for reducing nitrogen oxides in an effluent from the combustion of a carbonaceous fuel, there exists the present need to produce compositions which elicit still further reductions in the NO.sub.x concentrations in such effluents, especially at effluent temperatures which are practical for use with most common utility boilers.